smp_send_reschedule vs. smp_migrate_task

[Paul Mackerras <paulus@samba.org>]

I am looking at the updates for PPC that are needed because of the
changes to the scheduler in 2.5.x.  I need to implement
smp_migrate_task(), but I do not have another IPI easily available;
the Open PIC interrupt controller used in a lot of SMP PPC machines
supports 4 IPIs in hardware and we are already using all of them.

Thus I was thinking of using the same IPI for smp_migrate_task and
smp_send_reschedule.  The idea is that smp_send_reschedule(cpu) will
be effectively smp_migrate_task(cpu, NULL), and the code that receives
that IPI will check for the NULL and do set_need_resched() instead of
sched_task_migrated().

At present the i386 version of smp_migrate_task uses a single global
spinlock, thus only one task can be migrating at a time.  If I make
smp_send_reschedule and smp_migrate_task both use the same global
spinlock, is that likely to cause deadlocks or unacceptable
contention?  In fact it would not be hard to have a spinlock per cpu.
Would we ever be likely to do smp_migrate_task and set_need_resched
for the same target cpu at the same time?

Paul.